There were, however, cases, particularly in warfare against barbarians, where the enemy would not meet the imperial forces in the open field, but took up a defensive position on ground where legionaries could not be employed with success. In these circumstances the auxilia formed the first line and began the attack, and only if they were driven back and pursued by the enemy did the legions come into action. The battle of Mons Graupius, in 84, was conducted on these lines,[321] and similar tactics seem often to have been employed by Trajan in Dacia.[322] In general, however, the auxilia play a very secondary rôle; we do not hear either of the cavalry being used to strike the decisive blow after the manner of Alexander,[323] or of any such combination of archers and heavy infantry as we find in mediaeval warfare. The subject, however, is still obscure, and it is more satisfactory to turn to the part played by the auxilia in frontier defence, concerning which the archaeological research of the past twenty years has established more certain conclusions. In the frontier policy of the first two centuries we can trace two opposing tendencies at work, each of which is reflected in the disposition of the troops and the duties required of them. At the death of Augustus the Empire had as yet reached hardly any of its natural boundaries, although by means of the system of client kingdoms and ‘protected’ tribes it was asserting its claims and intentions in much the same fashion as the powers of modern Europe are doing in Africa to-day. The first century therefore witnessed on almost every frontier a period of expansion of greater or less duration, in which the sphere of direct administration was gradually pushed forward until some physical or political obstacle was reached which necessitated either a halt or a forward policy on a much larger scale. Throughout this period military operations were always imminent; in Britain, for example, between 50 and 85, the garrison marched out almost every spring, either for a campaign or a military demonstration. In winter, therefore, or in times of peace, the frontier armies were so disposed as to be able to take the offensive at a few days’ notice. The legions often lay in pairs, while many of the auxiliary regiments, instead of being scattered over a wide area, as was the case later, were concentrated at a few strategic points. The extent to which this system was adopted varied, of course, with local conditions, and a few regiments always occupied more isolated positions, but as a whole the auxilia of a province were far more easily mobilized than later when each regiment had its own _castellum_. On the Rhine frontier Haltern and Hofheim furnish examples of these large _hiberna_, dating from the beginning and the middle of the first century respectively,[324] and we find the same system continued for the defence of the Taunus district annexed by Domitian.[325] There is,